tag:blogger.com,1999:blog-8148573551417578681.post8686184667679076051..comments2024-03-27T19:47:13.475-07:00Comments on Dark Buzz: The Quantum Computing BubbleRogerhttp://www.blogger.com/profile/03474078324293158376noreply@blogger.comBlogger2125tag:blogger.com,1999:blog-8148573551417578681.post-83873319767989114682022-09-26T19:20:29.057-07:002022-09-26T19:20:29.057-07:00Roger, Scott was never challenged on the thermodyn...Roger, Scott was never challenged on the thermodynamic limits of QCs.<br /><br /> "All quantum computers are reversible computers and, as such, are constrained thermodynamically; the operating speed of a physically realisable reversible computer scales linearly with the amount of heat or entropy it generates (i.e., the more reversible a computer is, the slower it operates). Reversible computers require a small external force with each step to drive them forward. The speed of the step scales linearly with the applied force, which also scales linearly with the energy dissipated with each step. Thus the speed of operation scales linearly with the entropy released. For example the entropy released during adiabatic switching (an example of an implementation of reversible computing) scales linearly with the speed of operation: storing a bit dissipates CV2 *RC/t, of energy where t is the ramp time over which the voltage rises linearly from base. (“Instantaneous” storage releases CV2/2.) Brownian Turing machines also require a small driving force with each step; again their forward velocity scales with the energy dissipated. For a general quantum computer the Heisenberg uncertainty time-energy principle implies a bound of dE > h/dt, with each step (dE = energy released per step, dt = time for step to complete, h = Planck’s constant) and we would expect dE to be released as heat giving TdS = dE > h/(T dt), where T is the ambient temperature and dS is the entropy released per step.<br /><br />Another constraint is that a quantum computer must avoid decoherence, which implies that the total amount of entropy released over an entire computation must be O(k), k = Boltzmann’s constant. If dS >> k then the quantum computer decoheres into O(exp(dS/k)) independent microstates (by the Planck-Boltzmann law); any subsequent attempt by an external agency to read off the result of the computation will only be able to access O(exp(-dS/k)) of these microstates; garbage is read. For an operation that requires M steps this means that the averaged entropy release per step must be of O(k/M) or less for the quantum computer to function.<br /><br />Putting these two results together, we see that a quantum computer that completes processing in M steps takes at least hM2/ kT or O(M2) time to complete. This is a completely general result that applies to all quantum computers. No amount of shielding from external decoherence (which is believed to be their main problem) will get around this other problem which arises from internal decoherence. Lowering the temperature actually slows the computation down."<br />https://megasociety.org/noesis/179<br /><br />"It is shown that spontaneous symmetry breaking imposes a fundamental limit to the time that a system can stay quantum coherent...This universal timescale turns out to be t_spon ≃ 2πNħ/(k_B*T)." <br />https://arxiv.org/abs/cond-mat/0408357<br /><br />"one might wonder what help it could be to know that, in principle, a computer can be operated by spending zero energy but, in practice, this is obtained only under the condition that all the operations are performed adiabatically, i.e., extremely slowly."<br />https://www.mdpi.com/1099-4300/21/9/822/htmMD Coryhttps://www.blogger.com/profile/05342743632013663077noreply@blogger.comtag:blogger.com,1999:blog-8148573551417578681.post-43197233849775110912022-08-31T02:45:37.777-07:002022-08-31T02:45:37.777-07:00We should build a quantum computer because even if...We should build a quantum computer because even if it doesn't do anything useful... or even work, it will provide overpriced employment for experts who have nothing else more useful to do. Yay science.<br /><br />There is nothing more ridiculous than trying to to design a machine that has no specific applications. This isn't how engineering works.CFTnoreply@blogger.com